Low-temperature preserved goods such as frozen foods, etc. are generally denatured when once exposed to environments higher than the prescribed temperature, but there are cases wherein, when the denatured goods are again preserved at low temperatures, it becomes impossible to distinguish them from the undenatured goods by appearance. For previously checking the denaturation of low-temperature preserved goods which results from insufficient control on the marketing route, and supplying undenatured goods to consumers without anxiety, it is necessary that one can easily be judged that the goods have not been exposed on the way to temperatures higher than the prescribed one can easily be judged.
An object of the present invention is to provide a temperature-indicating apparatus for attaining the above purpose. More particularly, said object is to provide a temperature-indicating sheet which can be stuck directly on low-temperature preserved goods or can be inserted into or stuck on the package, and which, when the goods are exposed to temperatures higher than the prescribed one, can change in color according to the exposure temperature and time.
The thermochromic materials so far used include metal complex crystals and liquid crystals. These materials, however, are limited in the range of discoloration temperature and the kind of color, so that thermochromic compositions comprising organic electron-donative color precursor, electron-accepting organic compounds and alcohols, ester, etc., or those comprising organic electron-accepting color precursor, electron-donative organo-nitrogen compounds and alcohols, esters, etc., have been proposed for said materials. With these compositions, it is possible to freely select the discoloration temperature and the kind of color over a wide temperature range of from -100.degree. C. to more than 200.degree. C. But, these compositions do not come to provide such compositions which will color in change according to both the temperature and time as intended by the present invention.
Also, in the thermochromic compositions comprising combination of an organic electron-donative color precursor and an electron-accepting organic compound, there are proposed heat-integrating thermochromic compositions in which one of the above components is included in fine capsules, and which, since the fine capsules are broken when heated at a prescribed temperature for a prescribed time, change in color depending upon two factors of the time and temperature. In this case, the discoloration results from break of the fine capsules by heating to temperatures higher than room temperature, so that said compositions cannot be applied to low-temperature preserved goods as intended by the present invention. For temperature-indication at low temperatures, a method to detect the change in shape or color (a dye is used together) on melting of frozen brine, is proposed. In this case, however, the controllable temperature is 0.degree. C. or less, and also a change depending upon temperature and time cannot be detected, so that this method is not applicable to the present invention with an object to control common goods to be preserved at low temperatures including 0.degree. C. or more.
Temperature-indicating labels changing in color after a definite time has passed at a definite temperature, are also proposed (Japanese Pat. Appln. No. 64352/1983). It is however necessary to produce and store the label at temperatures lower than its discoloration temperature, so that the label is inconvenient in use.